Our objective is to understand the mechanisms responsible for the formation, maintenance and regression of the structure of corpora lutea and the mechanisms responsible for the initiation, maintenance and termination of their function. At present, the structure and function of corpora lutea appear to be regulated by factors from the pituitary gland, the uterus and the opposite ovary. We have examined the relationship of the number of corpora lutea and the weight of luteal tissue on the rate of progesterone secretion from the ovary of the pig. With this information, we have developed statistical methods which will permit us to study validly the influence of extra ovarian substances on luteal structure and function. We have developed radioimmunoassay (RIA) methods for the simultaneous measurement of the porcine gonadotropins, luteinizing hormone (LH), follicle stimulating hormone (FSH), prolactin (PRL), estradiol and progesterone in plasma. We have developed stalls for adult pigs (to which they readily adapt) and the surgical techniques required to provide an animal that can be employed while standing, unanesthetized and unconscious of being injected or of being sampled for blood. Catheters have been implanted in the vena cava. Correlations of venal caval concentrations of gonadotropins and steroids will provide a complete picture of the temporal relationships of gonadotropins and gonadal steroids during the various states of luteal development, steroidogenesis, maintenance and regression. Specific antisera to the gonadotropins and gonadal steroids will be employed to determine the cause and effect relationships of the hormones and their glands.